Displays such as a liquid crystal display panel (LCD), a digital micro mirror device (DMD), and a plasma display panel (PDP) are referred to as hold type displays. This means that a cathode-ray tube (CRT) provides impulse-type image output, while the LCD or the like maintains its image output state until the subsequent image is outputted, as shown in FIG. 1. On such hold type displays, when moving images are displayed, the images become unclear, unlike those on the CRT.
It has been conventionally considered that in a case where moving images are displayed, degradation in their image qualities is due to delay in a response to display by a display device. As studies on vision have proceeded in recent years, the response speed of the display device has been improved. Therefore, it has been found that even if the display device responds instantaneously, some degree of degradation in image qualities cannot be avoided. Such degradation in image qualities is referred to as hold blurring.
The hold blurring is due to a cumulative effect in a visual information processing system of human beings, and is a phenomenon which does not occur on an impulse output type display device such as a CRT, as shown in FIG. 2. A human being follows, when he or she observes a moving image, an object in the moving image with his or her eyes. At this time, the following speed of the eyeball cannot be rapidly changed. In a normal refresh period (17 ms) of the moving image, therefore, the eyeball moves at an approximately constant speed. In a hold type display device, however, the same image is displayed in a predetermined period (17 ms) at the same position, as shown in FIGS. 3 and 4(a). Therefore, the displayed image goes back relative to the position of the line of sight, so that an image with retreat movement is projected on the retina, as shown in FIGS. 4(b) and 5(a).
However, these occur in the preceding stage of the visual information processing system. The speed actually recognized is considerably slower than 17 ms. The images are integrated within a predetermined period, as shown in FIG. 5(b), and an image obtained by the integration is recognized as vision, as shown in FIG. 5(b). As a result, the recognized image is such a blurred image that trails which have moved on the retina for a predetermined period are overlapped with each other. The integration period is known as a Bloch's theorem, and is said to be approximately 50 ms to 80 ms. On the other hand, in the case of the CRT, images instantaneously displayed are only integrated, as shown in FIGS. 6(a) and 6(b). Even if the integration is performed by tracking, an image gone back toward the line of sight is not projected on the retina, so that a clear image is recognized.
The most general method of restraining hold blurring is to make the hold type display device analogous to the properties of the CRT. The CRT is an impulse output type display device, so that the above-mentioned problem does not occur. Therefore, the most effective method of improving moving image display characteristics of a hold type display is to perform intermittent display by applying light intermittently to a liquid crystal or a DMD, as shown in FIGS. 7(a) and 7(b) (see JP-A-09-325715: IPC G09F 9/35).
However, an irradiation time period is restricted to approximately 60% in reality. As a result, the luminance of a liquid crystal is also decreased to 60%. It cannot be said that the restriction of the irradiation time period to 60% is sufficient for an improvement in degradation in image qualities by a defect of a hold type display. Naturally, the shorter the irradiation time period is made, the greater the improving effect becomes. However, the rise in cost due to the necessity of a bright backlight, the increase in the size of a power supply or the like, and so on present a great problem.
Such a method is difficult to realize by a liquid crystal projector employing a high-output lamp. The flashing of the lamp heavily damages the lamp and affects the life. Even when light is shuttered, a large part of the shuttered light becomes heat, so that a problem of radiation of heat arises.
There is a method of separating backlights in a direct view type liquid crystal display and scrolling the flashing of each of the backlights to obtain the same effect as that in shuttering (see JP-A-2001-235720: IPC G02F 1/133). This method also has the disadvantages of making it difficult to adjust synchronization and reducing display luminance, similarly to the above-mentioned method. Further, a circuit is increased in size, and the manufacturing cost thereof is significantly increased.
A method of inserting a black level display for each predetermined time period in the case of display on a hold type display has been proposed (see JP-A-11-109921: IPC G09F 9/36). Generally, the predetermined time period corresponds to a refreshment period of a frame. For example, a method of displaying an image during a period of 9 ms in a period of 17 ms and displaying black during the remaining period of 8 ms is employed. When the method is used, the synchronization is stabilized. However, the reduction in the display luminance can not be avoided. In the case of a liquid crystal or the like, a device which is high in response speed is required.
An example of a method of restraining hold blurring other than intermittent display is frame rate conversion. This method presents intermediate image in a period of 17 ms, because blurring is recognized as a result of presentation of the same image in the period in the case of the hold type display. Specifically, 60 images each corresponding to an intermediate part of the image are produced on the basis of original 60 images in outputting 60 Hz images. Then 120 images are displayed as 120 Hz images. As a result, a period during which the same image is presented, which is a cause of hold blurring, is halved. Consequently, the recognized blurring is halved, thereby obtaining clearer images than those in a case where the 60 Hz images are displayed.
However, this method requires some degree of accuracy in the intermediate images. In the current technique, such intermediate images cannot be reliably produced.
With regard to a liquid crystal projector, JP-A-2002-6815 (G09G 3/36) discloses a method of scrolling light onto a panel using a condenser mirror. In a condensing system (a polygonal mirror) herein disclosed, however, light is scrolled by a reflecting function. When a projector is constructed, therefore, an optical system is made significantly large.
In view of the foregoing circumstances, an object of the present invention is to provide a projection type video display capable of improving degradation in image quality, in a case where a moving image is displayed, which is referred to as hold blurring, by scrolling light onto a hold type display element.